If you look in the owners manual it says to use 89 or higher. So I wouldnt say its bad but its not good for it and thats prolly why you are getting a knock. All I ever run is 93 from shell, but thats preference though.
Wow always run what the owners manual recommend anything lower an you can get pinging/ knocking which is bad for your engine can destroy it in the long run anything over is 100 percent a waste of money and your not getting anything out of it
Unless you have a tune of course